1. Field of the Invention
The present invention relates to an optical amplifier and a fiber module for optical amplification. The present invention relates also to an apparatus usable as a white light source provided by the optical amplifier according to the present invention.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) silica optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put into practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying an optical signal or signal light has been put into practical use.
An optical amplifier known in the related art includes an optical amplifying medium to which signal light to be amplified is supplied and a means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light.
For example, an erbium doped fiber amplifier (EDFA) has already been developed to amplify signal light in a 1.55 .mu.m band where the loss in a silica fiber is low. The EDFA includes an erbium doped fiber (EDF) as the optical amplifying medium and a pump light source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band, a gain band including a wavelength of 1.55 .mu.m can be obtained.
Further, another type of the optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of combining WDM and an optical amplifier to construct a system, a transmission distance is limited by the gain characteristic (gain dependence on wavelength characteristic) of the optical amplifier, which is represented by a gain deviation or gain tilt. For example, in an EDFA, a gain tilt is generated at wavelengths near 1.55 .mu.m. If the gain deviation is accumulated in a chain of plural cascaded EDFAs, an optical SNR (signal-to-noise ratio) in a channel included in a small-gain band is deteriorated, for example.
The optical amplifying medium containing a rare earth (element) as a dopant is classified into a silica fiber obtained by doping a rare earth in silica glass having a primary component similar to that of a transmission fiber and a nonsilica fiber obtained by doping a rare earth in nonsilica glass such as fluoride glass and tellurite glass. The silica fiber has an advantage such that it is easy to handle, but has an intrinsic problem such that a relatively large gain deviation tends to be generated. In contrast, the nonsilica fiber has an advantage such that a gain deviation is hardly generated, but has a disadvantage such that it is not easy to handle. In a fluoride glass fiber, for example, it has a low melting point, and fusion splicing to a silica fiber is therefore difficult. Furthermore, the mechanical strength of the fluoride glass fiber is low, so that its reliability related to a breakage life or the like is low. In addition, since the fluoride glass fiber has a deliquescent property, an advanced packaging technique is required.
Further, generally in a rare earth doped silica fiber or nonsilica fiber, the mode field diameter (MFD) of these fibers is set smaller than that of a transmission fiber, so as to increase a pumping efficiency. Accordingly, it is sometimes difficult to effectively supply to the fiber a pump light beam for pumping the fiber, resulting in a reduction in pumping efficiency.